1. Field of the Invention
The present invention relates to a light transmitting and receiving device and, in particular, to the one suitable for performing two-way optical communication by transmitting and receiving light beams in the outdoor atmosphere as a transmission medium.
2. Description of the Related Art
A variety of light transmitting and receiving devices for transmitting and receiving optical beams through the outdoor atmosphere as a transmission medium have been proposed.
In the optical communication system disclosed in Japanese Patent Laid-Open No. 5-134207, two identically constructed optical communication devices installed facing each other with a distance allowed therebetween perform two-way optical communication.
FIG. 24 shows a major portion of one conventional optical communication device. This optical communication device comprises light projecting means having a light emitting device 101 for generating a signal light and a positive-powered projection lens 102, light receiving means having a light receiving device 103 for receiving a signal light and a positive-powered light receiving lens 104, a polarizing beam splitter 105 which reflects a projected light beam while receiving a received light beam, and a beam expander 106 which expands a projected light beam LA while converging a received light beam LB. The beam expander 106 has a negative-powered lens unit 107 and a positive-powered lens unit 108.
The polarizing beam splitter 105 is located where the optical axes of the light projection lens 102 and the light receiving lens 104 intersect each other, and the beam expander 106 is arranged to the transmission side of the polarizing beam splitter 105 for the projected light beam.
A laser diode is used for the light emitting device 101 for generating the signal light, and the polarizing beam splitter 105 is a rectangularly parallelopipedal or cubic polarizing beam splitter having a beam splitting surface 105a of a deposited dielectric multilayer which reflects most of the s-polarized light beam while transmitting most of the p-polarized light beam.
To achieve efficient light transmission and reception using the polarizing beam splitter 105, a light beam LB received from an opposing light transmitting and receiving device B (not shown) is designed to be p-polarized when a transmitted light LA from the light transmitting and receiving device A shown in FIG. 24 is s-polarized.
The s-polarized light beam of the light beam emitted from the light emitting device 101 for generating the signal light becomes a substantially parallel light beam, is mostly reflected by the beam splitting surface 105a of the polarizing beam splitter 105, and is transmitted as the transmitted light beam LA from the light transmitting and receiving device A to the light transmitting and receiving device B.
The (p-polarized) light beam projected by the light transmitting and receiving device B is incident on the light transmitting and receiving device A as the received light beam LB, and is mostly transmitted through the beam splitting surface 105a of the polarizing beam splitter 105, and reaches the light receiving device 103 for detecting the signal light.
Two-way optical communication is performed for light transmission and reception in this way in the above system.
In the optical communication device constructed as shown in FIG. 24, the light beam projected from the light projection lens 102 is substantially vertically incident, in the form of a substantially parallel light beam, on a transmission and reception surface 109 of the polarizing beam splitter 105. For this reason, a light beam 110 reflected from the transmission and reception surface 109, out of the projected light beam, is introduced in whole (i.e., in its entirety) in an effective light receiving surface of the light receiving device 103 as noise light.
This noise light is now referred to as a first noise light. As shown in FIG. 25, the light beam 112 transmitted through the beam splitting surface 105a reaches and is reflected by a surface 111 shown as a top surface, is reflected from the beam splitting surface 109a, travels to the light receiving lens 104, and is introduced in whole in the effective light receiving surface of the light receiving device 103 for signal light detection as noise light. This noise light is here referred to as a second noise light.
The first and second noise light rays are the cause for a so-called cross-talk, and degrade the accuracy of optical communication. To cope with the second noise light, the surface 111 is conventionally subjected to antireflection process by grinding and then applying a black coating thereon. To cope with the first noise light, the only available antireflection process is applying an antireflection coating onto the transmission and reception surface 109. The antireflection coating cannot reduce reflectance to zero and cross-talk still persists.
Simply tilting the polarizing beam splitter 105 of the conventional structure cannot improve the situation in which two noise light rays return to the light receiving lens 104. If an attempt is made to tilt only the transmission and reception surface 109 of the polarizing beam splitter 105, the optical axis of projection and the optical axis of reception fail to intersect each other at a right angle, and in such a case, particular consideration needs to be given to the design of a lens barrel, requiring a complex component design.
It is an object of the present invention to provide a light transmitting and receiving device for use as an optical communication device for accurate optical communication in which at least first noise light or second noise light is efficiently prevented from entering a light receiving element by setting up the appropriate form of a polarizing beam splitter through which transmission and reception of beams are performed for two-way optical communication.
Accordingly, in one aspect, the light transmitting and receiving device of this invention, through the polarizing beam splitter of a transparent body including a beam splitting surface therewithin, projects a light beam from light projecting means in a predetermined direction and receives a light beam in the predetermined direction at light receiving means, wherein the polarizing beam splitter is designed such that a light beam reflected from one of the surfaces constituting the polarizing beam splitter, out of the light beam from the light projecting means, is introduced into the light receiving means via the beam splitting surface at an inclination with respect to the optical axis of the light receiving means.
More particularly, the light transmitting and receiving device of the present invention preferably comprises a light projecting unit having an optical axis of projection, a light receiving unit having an optical axis of reception, and an optical member, disposed where the optical axis of projection intersects the optical axis of reception, including therewithin a light beam splitter which reflects one of the light beam of the light projecting unit and the light beam of the light receiving unit while transmitting therethrough the other of the light beam of the light projecting unit and the light beam of the light receiving unit, and having a transmission surface from which the light beam of the light projecting unit is transmitted and a surface opposing the transmission surface, wherein the transmission surface and the opposing surface are mutually in parallel and are inclined to the principal ray of the transmitted light beam from the transmission surface.
In the light transmitting and receiving device, the light projecting unit preferably comprises a light source for emitting a polarized light beam, and the beam splitter is a polarizing beam splitter.
Preferably, the optical axis of projection and the optical axis of reception are perpendicular to each other.
Preferably, the optical member has an entrance surface on which the light beam from the light projecting unit is incident and a surface opposing the entrance surface, and either the entrance surface or the opposing surface is parallelogrammatic in shape.
Preferably, the optical member has an entrance surface on which the light beam from the light projecting unit is incident and a surface opposing the entrance surface, and either the entrance surface or the opposing surface is rhombic (i.e., in the shape of a rhombus) in shape.
Preferably, the transmission surface of the optical member is rectangular in shape.
Preferably, the transmission surface of the optical member is square in shape.
Preferably, the optical member has a side surface in parallel with the plane in which both the optical axis of reception and the optical axis of projection lie, and the side surface is parallelogrammatic in shape.
Preferably, the optical member has a side surface in parallel with the plane in which both the optical axis of reception and the optical axis of projection lie, and the side surface is of a rhombus in shape.
Preferably, the transmission surface of the optical member is rectangular in shape.
Preferably, the transmission surface of the optical member is square in shape.
In view of the foregoing, in one aspect, the present invention relates to a light transmitting and receiving device comprising (a) a light projecting unit having an optical axis of projection; (b) a light receiving unit having an optical axis of reception; and (c) an optical member, disposed where the optical axis of projection intersects the optical axis of reception, the optical member including therewithin a light beam splitter which reflects one of a light beam from the light projecting unit and a light beam travelling to the light receiving unit while transmitting therethrough the other of the light beam from the light projecting unit and the light beam travelling to the light receiving unit, wherein the optical member has a transmission surface from which the light beam from the light projecting unit is transmitted and a surface opposing the transmission surface, wherein the transmission surface and the opposing surface are mutually in parallel and are inclined with respect to the principal ray of the transmitted light beam from the transmission surface.
In another aspect, the present invention relates to an optical member for use with a light transmitting and receiving apparatus, the apparatus having (a) light transmitting means for transmitting light, the light transmitting means having an optical axis of projection, and (b) light receiving means for receiving light, the light receiving means having an optical axis of reception, wherein the optical member is disposed where the optical axis of projection and the optical axis of reception intersect, the optical member comprising (a) a top surface; (b) a bottom surface opposing and parallel to the top surface; (c) a light entrance surface for receiving light from the light transmitting means; (d) a side surface opposing and parallel to the light entrance surface; (e) a light transmission and reception surface extending between the light entrance surface and the side surface; (f) a light exit surface opposing and parallel to the light transmission and reception surface; and (g) a polarizing light beam splitting surface inside the optical member, wherein each of the top surface, the bottom surface, the light entrance surface, the side surface, the light transmission and reception surface, and the light exit surface is an exterior surface of the optical member, wherein light from the light projecting means enters the light entrance surface, is reflected by the polarizing light beam splitting surface, and exits from the optical member through the light transmission and reception surface, while incoming light passes through the light transmission and reception surface, is transmitted through the polarizing light beam splitting surface, and exits through the light exit surface to the light receiving means, and wherein the central beam of the light exiting through the light transmission and reception surface is inclined with respect to a line normal to the light transmission and reception surface. Preferably, the optical member is a polarizing beam splitter, most preferably one which has a surface that reflects light having one polarization while transmitting therethrough light having a different polarization.
These and other aspects, objects, advantages, and features of the present invention will become apparent from the following detailed description of preferred embodiments thereof taken in connection with the accompanying drawings.